Slack is giving other companies a new way to integrate their content into the startup's business chat app.

Outside developers can now incorporate a button into their own apps and sites called "Add to Slack." The button lets users of those apps share content or otherwise post in Slack, but without leaving the other apps.

Think of it as the Facebook "Like" plugin, but instead of sharing content back to Facebook, you're sharing content into a Slack chat.

More than a dozen third-party apps have already added the button to their sites, including Nuzzel, a companion app for Twitter; InVision, a prototyping and design app; and cloud storage service Box.

Box's new integration with Slack, for instance, lets Box users import a file from Box or share a link to a file in Slack. InVision's integration means designers can share designs with their team directly to a Slack channel.

Slack is also releasing documentation on the new button so that outside developers can incorporate it into their apps.

The new function may help Slack keep growing its users at a time when it faces competition in the business collaboration space from established players like Google and Microsoft, as well as from other startups.

Over 80 other applications have been already integrated with Slack using its APIs, including Twitter, Google Drive and Dropbox.

With the feature announced Tuesday, Slack said it will increase the number of integrations allowed to free users of its app, from five to 10. Users of Slack's paid versions get unlimited external integrations.

Slack, which launched in early 2014, claims to have over 1.1 million daily active users. The company says it had roughly 125,000 users a year ago.